Generally, such systems attempt to depict or suggest three-dimensional information which the human ear is able to break down. This can be achieved by the reproduction of two or more differently constituted final signals, by the addition of artificial early reflections or artificial diffuse sound or by the simulation of audio circumstances relating to the human head by means of HRTF, alternatively. These approaches to a solution are used particularly in order to convert monophonic audio signals into audio signals which convey to the ear an actual or fictitious three-dimensionality. Such methods are referred to as “pseudo-stereophonic”.
In comparison with conventional stereo signals, pseudo-stereophonic signals usually exhibit deficiencies. In particular, psychoacoustic reasons mean that the localizability of the sound sources, for example in the case of methods which distribute the frequency spectrum with different phase shifts over the final signals, is restricted. The application of propagation time differences also normally results in inconsistent localization for the same reasons. Artificial reverberation, likewise for psychoacoustic reasons, prompts fatigue phenomena in the listener. A series of proposals have been made, particularly by Gerzon (see below), which are intended to eliminate such inconsistencies in the stereophonic depiction of sound sources. Reproduction of the original three-dimensional circumstances, as conventional stereo signals aim to depict, does not usually occur even in complex applications, however.
In particular, pseudo-stereophony based on the simulation of intensity-stereophonic methods has the particular problem that a monophonic audio signal based on a figure-of-eight directivity pattern cannot be stereophonized, on account of the nondepiction of sound which is incident from the side.
The prior art is formed by the following documents:
U.S. Pat. No. 5,173,944 considers signals, obtained at constant azimuth of 90 degrees, 120 degrees, 240 degrees and 270 degrees by means of HRTF from the differently delayed but uniformly amplified fundamental signal, which are overlaid on the fundamental signal. In this case, level and propagation time corrections remain independent of the original recording situation.
U.S. Pat. No. 6,636,608 proposes phase shifts, determined on the basis of frequency, in the mono signal to be stereophonized which are overlaid on the original monophonic audio signal both in the left-hand and in the right-hand channel with different gains—which are likewise independent of the recording situation!
The aforementioned document U.S. Pat. No. 5,671,287 (Gerzon) improves a method proposed by Orban (which takes a monophonic audio signal and obtains a summed signal and a difference signal which have frequency-dependent phase shifts—regardless of the recording situation!), these improvements likewise being based on frequency-dependent phase shifts or on a gain—regardless of the recording situation!—given slightly altered formation of the summed and difference signals.
The applicant's own European application No. 06008455.5 proposes methodical consideration of the manually or metrologically ascertained angle φ between main axis and sound source using propagation time and level differences which are dependent on the angle φ. If the angle φ is equal to zero, however, compatible stereophonic depiction is not possible.
The invention explained below is intended to be a significant improvement in the stereophonic reproduction of a monophonic depicted sound source, taking account of the recording situation. In addition, a reliable method of stereophonization is intended to be provided for the aforementioned figure-of-eight directivity pattern, which has to date been problematical for intensity-stereophonic simulations. Subsequently, the aim is to allow compatible stereophonic depiction even for the case in which the angle φ between main axis and sound source is equal to zero.
The subject matter of the invention can be presented as follows:
The technical solution-proposed in the applicant's own European application No. 06008455.5—of methodical consideration of the angle φ between main axis and sound source using propagation time and level differences which are dependent on the angle φinvolves MS matrixing, where the following relationships apply to input signals M and S and resultant signals L and R:
                    L        =                              (                          M              +              S                        )                    *                      1                          2                                                          (        1        )                                R        =                              (                          M              -              S                        )                    *                      1                          2                                                          (        2        )            
The classic S signal-which is specific to MS engineering—has a figure-of-eight directivity pattern, said signal being offset from the M signal by 90 degrees to the left. If the level of the S signal is now increased in comparison with the M signal, what is known as the opening angle 2α (which is obtained from the points of intersection of the overlapping polar diagrams for the M system and the S system and—like the figure-of-eight directivity pattern of the S system—is always situated symmetrically with respect to the main axis of the M signal) is reduced to an increasing extent.
In a first step, it is possible to parameterize a fictitious opening angle 2α even in an arrangement or a method which takes account of the angle φ between the main axis of the monophonic signal and the sound source. The calculated simulated side signal is then dependent both on the angle φ and on half the fictitious opening angle α.
In a second step, gain factors are applied only to the signals which produce the side signal when summed.
In a third step, the angle-dependent polar interval f describing the directivity pattern of the M signal is parameterized. It is therefore now possible to stereophonize monophonic signals of arbitrary directivity pattern taking account of a fictitious opening angle 2α.